Communications networks require accurate time synchronization of clocks distributed across the network in order to properly operate. In a communications network comprising one or more client networks connected by an OTN communications network packet network, time synchronization information is carried across the OTN network by transmitting packets containing timestamp information. Timestamps are generated by a master (server) that has access to an accurate time reference, such as GPS, and transmitted across the OTN network to a receiving system (slave). The slave runs an algorithm that recovers the timing based on the timestamp of the packets and on their arrival times. When time synchronization is requested, a two-way timing protocol is mandatory where the transfer delay from master to slave shall be calculated. One fundamental assumption with this approach is that the master-to-slave and slave-to-master propagation times (path delays) across the OTN network are equal. This means that any asymmetry in the OTN network will significantly reduce the accuracy of the time synchronization between the master and slave clocks.
The solution currently proposed in the standards to allow accurate time sync transmission over OTN networks is to use the precision time protocol (PTP) defined in IEEE 1588 and to process PTP packets at each node in a path across the OTN network, by providing Transparent Clock or Boundary Clock functions at each node. Several methods have been proposed for implementing this approach including the use of the OTN packet overhead to transmit the PTP timing data, use of an optical supervisory, OSC, channel of the OTN network to transmit PTP data packets, and use of a general communications channel, GCC, channel of the OTN network to transmit PTP data packets.
Processing PTP data packets at each node during transmission across an OTN network may make it difficult to deal with multiple time synchronisation traffic flows, for example when multiple client networks operated by different network operators, share a single OTN network. It also requires that each node in the OTN network must be configured to support IEEE 1588.